The present invention relates generally to a method of fabricating plastic encapsulated integrated circuits and more specifically to flip chip devices with heat spreaders.
Integrated circuit chips are being fabricated with ever smaller geometries and higher circuit densities. The power consumption associated with these more dense circuits has also increased, thereby increasing the thermal transport requirements of the package housing the chip. Further, as the volume of integrated circuits has grown, more cost-effective packages are being developed. In order to meet that need, highly automated techniques for encapsulating the devices in plastic molding compounds continues to evolve. However, thermal properties of such packages has been limited, and has led to attempts to improve the thermal performance.
In conventional thermal transport arrangements of plastic molded packages, a thermoconductive structure has been positioned close to or against the integrated circuit chip, and has been partially of fully encapsulated by a plastic molding compound which is filled with an electrically insulating particles. Package designs make use of conduction, convection and radiation to spread and remove the heat from the circuit junction. In some devices, as shown in FIG. 1a, one surface of the heat sink 101 has been exposed through the package encapsulation 102 in order to provide a direct path for transfer of heat from the integrated circuit chip 105 assembled on the die pad of a metal lead frame 108 to a printed circuit board (not shown). Exposed heat sink packages have been developed where the heat path has been directed through the bottom of the package to board, or through the top of the package to the ambient or to an attached heat sink. In cases of more moderate thermal requirements, the chip has been attached to a thermally conductive structure simply to spread the heat over an area larger than that of the chip, and the heat spreader encapsulated within the package. No precise rules have been developed to define the differences, but in general, heat spreaders and heat sinks differ in that heat spreaders may be thinner than heat sinks, and spreaders may be encapsulated within the package, as opposed to having an a major surface external to the package.
Unfortunately, manufacturing methods for thermally enhanced packages, as shown in FIG. 1a, have not lent themselves to the high level of automation needed for assembly of lower cost plastic packages. A widely used method for attaching heat spreaders has been to place a single metal structure 101, such as that shown in FIG. 1b into each cavity of a mold, position a strip of lead frames with the attached integrated circuit device in the mold so that the die paddle or the chip itself comes into contact with the metal heat spreader, and to inject a thermosetting plastic molding compound to fill the mold. The method requires placing an individual heat spreader or sink in each cavity of the mold, thereby adding extra process steps, and the cycle time associated with molding a plurality of devices in strip form. Further, because the individual heat spreaders lack a clamping location, they must be of sufficient weight and thickness to hold them securely in place during molding. Despite designing the heat spreaders with positioning fingers 111 and apertures 112 for locking the mold compound, the heat spreaders often move during the molding process, and molding compound is forced in an uncontrolled manner between the heat spreader and die pad, or onto the heat spreader outside the package.
As area array and flip chip integrated circuit packages have evolved, it has become even more difficulty to economically fabricate thermally enhanced packages. With flip chip interconnections, the integrated circuit has a plurality of solder balls positioned on the active surface of the device which are attached to receiving pads on a substrate. The location of the junction on the circuit generating heat frequently does not coincide with that of the solder balls and therefore, thermal transport may be much less effective than with conventionally packaged devices where the heat transfer path is through the silicon and out the backside of the chip into a metallic lead frame. Further, the substrates for flip chip devices are seldom good thermal conductors. Elaborate schemes for attaching heat sinks have been developed for very high power devices, but for those mid-power devices in the range of 1.5 to 3.5 Watts, housed in thin, molded plastic packages, an automated method of heat spreader attachment is lacking.
Not only has power consumption of integrated circuits increased to the point that greater than 1 watt devices are very typical, but package size has become much smaller. The area has been decreased by replacing external leads with solder ball connections of area array packages, and the thickness has decreased so that a typical package is 1 mm or less. With decreasing package size, the difficulty in providing techniques for removing heat has increased considerably.
Yet another issue in fabricating plastic molded thermally enhanced integrated circuit packages is associated with the molding process itself. In thermally enhanced packages, as in FIG. 1a, high pressure within the mold cavity during encapsulation can cause warping of the heat spreaders, resulting in uncontrolled spacing between the die and thermal conductor. Further, if the heat spreader or sink has a major surface exposed for thermal transport, mold compound flashing or resin bleed onto the exposed surface can again result in inconsistent quality. Inconsistent quality in molded thermally enhanced packages may lead to yet another reliability problem, that is that contaminants and moisture may pass along the interface between the large metal pieces and the molded plastic due to poor sealing, and may migrate to the die surface where they contribute to leakage or corrosion failures. Various mechanical locking, as well as chemical adhesion promoting efforts have been proposed to minimize the problem.
Clearly a need exists for a cost effective method to fabricate reliable thermally enhanced packages where the method is consistent with the high level of automation associated with assembly of plastic packages, and in particular a need exists for a method to fabricate thermally enhanced packages for flip chip bonded devices.
In accordance with the invention, there is provided a method of fabricating a plurality of flip chip integrated circuit packages having heat spreaders assembled in strip format, and including the first step of providing the heat spreader strip assemblage. The heat spreader strip includes a series of heat spreaders connected to side rails by pillar shaped reduced cross section connectors which are readily severed to separate the packages after molding.
In a preferred embodiment of the current invention, integrated circuit packages having flip chips bonded to area array substrates are thermally enhanced by encapsulating heat spreaders in strip format. A strip of substrates and a corresponding strip of heat spreaders are positioned in a mold press, clamped at the reduced cross section connector on the heat spreader strip, and plastic encapsulant introduced. After the molding plastic has solidified, the strip is removed, and the individual packages separated at the reduced cross section contact area. Polymeric substrate strip are cut to complete separating the assembled packages.
The reduced cross section connector on the heat spreader strip minimizes the area of metal to plastic interface where it exits the plastic encapsulation, thereby minimizing the possibility of ingress of contaminants. To further enhance adhesion between metal and plastic, and to minimize the cross sectional area of metal to be severed in singulating the packages, each connector strap incorporates an elongated slit to decrease the area even more, and to provide a xe2x80x9cuxe2x80x9d shaped metal section within the package after being separated at the mid section of the slit. The xe2x80x9cuxe2x80x9d shaped inclusion in the packages serves as a dam against ingress of contamination.
The heat spreader strip is designed with openings in one side rail providing an area for flow of molding compound into each package cavity. The opposite side rail is continuous to provide the strip continuity. The continuous strip is designed with small apertures within the cross sectional area of the rail at specific locations to mate with pin locators in the mold.
The heat spreaders are offset from the plane of the side rails by sufficient height to allow clearance for the flip chip integrated circuit with solder or other connecting bumps. The offset occurs within the connector straps.
Heat spreaders of the preferred embodiment are perforated with apertures sufficiently large enough to allow molding compound to flow through and around the apertures. The perforations provide multiple locking mechanisms between the two materials, and allow the encapsulant to completely fill the space between the heat spreader and chip back surface, thereby eliminating thermally insulating air pockets.
The preferred embodiment of the current invention further includes a plurality of small cone shaped protrusions on the heat spreaders which rest against the mold cavity, thereby maintaining the position of the heat spreader within the package vertically, and yet creating only very small areas of metal exposure in the encapsulation at the point of the cones, again minimizing paths for ingress of contaminants.
Alternate embodiments include assembly of packages with exposed heat spreaders, and using heat spreader strips having solid rather than perforated heat spreaders. Further, the strip heat spreader technology is amenable to fabricating high-density designs with multiple packages across the width of a strip by providing a means for positioning mold compound runners. Leaded plastic packages are assembled using heat spreaders in strip format by providing electrically insulating connectors between the heat spreader and side rails to eliminate shorting to the lead frame.
Heat spreader strip designs and methods of use support automation of fabricating molded flip chip packages, with the accompanying cost reductions in labor and cycle time, while providing reliable and predictable thermally enhanced packages.
The drawings constitute a part of this specification and include exemplary embodiments of the invention which may be embodied in various forms. It is to be understood that in some instances aspects of the invention may be shown exaggerated or enlarged to facilitate understanding of the invention.